1. Field of the Invention
Aspects of the present invention relate to a passive cooling system for a fuel cell stack, and more particularly, to a passive cooling system for a fuel cell stack in which a primary coolant that circulates in cooling plates is circulated without pumping.
2. Description of the Related Art
A fuel cell is an electrical generation system that transforms chemical energy directly into electrical energy through a chemical reaction between oxygen introduced to the system and hydrogen contained in a hydrocarbon group material such as methanol, ethanol, or natural gas.
A polymer electrolyte membrane fuel cell (PEMFC) has advantages of superior output, low operating temperature, rapid starting, and a short response time compared to other fuel cells, and currently is the preferred fuel cell for automotive, portable, residential and small commercial applications.
Conventional PEMFCs are mainly operated at low temperatures, that is, below 100° C., e.g., about 80° C., because of a drying problem in a polymer electrolyte membrane. Because of their low operating temperatures, conventional PEMFCs also have the following problems. That is, a hydrogen-rich gas, which is a representative fuel for the PEMFCs, is obtained by reforming an organic fuel such as natural gas or methanol. However, the hydrogen-rich gas includes not only carbon dioxide, but also carbon monoxide as byproducts from the reforming process. The carbon monoxide may poison catalysts contained in a cathode and an anode. When the catalysts are poisoned, the electrochemical activity of the catalysts is greatly reduced, and accordingly the operating efficiency and lifetime of the PEMFCs are seriously reduced. The lower the operating temperature of the PEMFCs, the more severe is catalyst poisoning by carbon.
When the operating temperature of the PEMFCs increases to over 130° C., the fuel reformer can be miniaturized and a cooling apparatus can be simplified since catalyst poisoning by carbon monoxide can be avoided and the operating temperature of the PEMFCs can be readily controlled, thereby miniaturizing the overall PEMFC system.
A fuel cell system in which the fuel cell is cooled using cooling water having a temperature of approximately 70 to 80° C. has been disclosed in U.S. Pat. No. 7,052,790. This fuel cell system includes a cooling water pump that supplies cooling water from a cooling water tank to a fuel cell stack.
However, a fuel cell system cannot be reduced in size because of the volume of the cooling system when the fuel cell stack has a cooling water pump.